sore triceps after back work???

i feel like i should know the answer to this by now, but every once in a while ill hit back while my triceps are still sore (did tri's saturday, then back yesterday) and for some reason i feel like ive worked them out all over again... weighted pullups really made them sore while i was working out (kept trying to stretch them to no avail) and today when i woke up they were more sore than they were yesterday when i woke up... so im having to scrap my chest workout for the day.... anyone know why this is?